1. Field of the Invention
The present invention relates to a passive restraint, such as an air bag unit, used for protecting a driver and passengers in a vehicle when the vehicle crashes. More specifically, the invention pertains to an apparatus for controlling activation of such a passive restraint as well as to a method of the same. The present invention further relates to an apparatus for determining an amount of deformation of a vehicle, which crashes into a collision object.
2. Description of the Prior Art
One example of known apparatuses for controlling activation of a passive restraint is one for controlling ignition of a squib included in an air bag unit. In the air bag unit, a gas-generating agent is ignited with a squib in an inflator to evolve a gas from the inflator and inflate a bag with the gas, in order to protect a driver and passengers from the impact of collision.
Conventional apparatuses for controlling ignition of a squib in such an air bag unit are generally provided with a retardation sensor (hereinafter may be referred to as G sensor) for measuring a retardation applied to a vehicle (especially, a retardation in a longitudinal direction of a vehicle). Ignition control is carried out according to the retardation measured by the G sensor. Proposed ignition control processes based on the retardation include a method using the value of retardation itself, another method using the integral value obtained by integrating the retardation once, and still another method using the second integral value obtained by integrating the retardation twice.
The method of controlling ignition based on the second integral value of the retardation (herein "the second integral value of the retardation" means "the double integral value of the retardation") is, for example, disclosed in JAPANESE PATENT LAID-OPEN GAZETTE No. 6-107113. The proposed method determines an ignition timing of a squib based on the second integral value of the retardation.
The second integral value obtained by integrating the retardation twice over time generally represents the amount of displacement of a non-fixed object in a vehicle relative to the vehicle. By way of example, when an excess retardation is applied to the vehicle in its longitudinal direction, a driver and other passengers in the vehicle are pulled and moved forward by the force of inertia. The second integral value corresponds to a relative distance by which the driver or the passenger moves relative to the vehicle upon impact. The proposed method accordingly compares the second integral value of the retardation with an appropriate threshold value. When the second integral value exceeds the threshold value, the gas-generating agent in the air bag unit is ignited with the squib. The conventional method using the second integral value of the retardation representing the relative distance by which the driver or the passenger moves determines an ignition timing of the squib by taking into account the relative distance and the expanded volume of the bag.
In air bag units, it is extremely important to determine whether the bag is to be opened or not (that is, whether the gas-generating agent is to be ignited with the squib or not) according to the driving condition of the vehicle. The proposed method using the second integral value of the retardation, however, may not adequately determine whether the gas-generating agent is to be ignited with the squib or not, while determining the appropriate ignition timing of the squib as discussed below.
FIGS. 10(a) through 10(c) are characteristic charts showing the retardation G measured by a G sensor as well as the integral value v and the second integral value S of the retardation G, which are all plotted against the time, upon collision of a vehicle. FIGS. 10(a), 10(b), and 10(c) respectively show the time-based variations in retardation G, integral value v of the retardation G, and second integral value S of the retardation G. The origin of the time scale `t` is located at a time point when the vehicle crashes into the collision object.
Upon collision of the vehicle, the second integral value S of the retardation G draws a time-based characteristic curve, which is substantially approximated by a quadratic function starting from the origin, as shown in FIG. 10(c). When the time `t` is infinity, the second integral value also becomes infinity. No matter what threshold value is set for the second integral value S, the second integral value S certainly exceeds the threshold value after a lapse of time period. Even if it has been set to prohibit ignition of the gas-generating agent with the squib when the second integral value S is equal to or less than an appropriate threshold value, since the second integral value S exceeds the threshold value in due course, it is rather difficult to appropriately determine whether the gas-generating agent is to be ignited with the squib or not (that is, whether the bag is to be opened or not) using only the second integral value S.
The conventional method accordingly requires a different operation value other than the second integral value of the retardation for determining whether ignition is to be carried out or not.
Upon collision of the vehicle, unlike the second integral value S, the retardation G does not increase infinitely but is limited to be not greater than a specified value, although significantly varying as shown in FIG. 10(a). Among the known processes for controlling ignition of the squib, the method using the retardation G itself can thus determine whether ignition is to be carried out or not (that is, whether the bag is to be opened or not).
There are, however, some cases of collision, in which the method using the retardation G only can not effectively determine the ignition timing of the squib.
FIGS. 11(a) through 11(c) illustrate the concept of a pole crash and an under-ride collision of a vehicle as well as a time-based variation in retardation G of the vehicle upon collision of the vehicle. FIG. 11(a) shows a case in which a front part of a vehicle 100 crashes into a pole, such as a utility pole 102; FIG. 11(b) shows a case of an under-ride collision in which the front part of the vehicle 100 crashes into, for example, a heavy-duty truck 104; and FIG. 11(c) shows the retardation G plotted against the time in the pole crash or in the under-ride collision.
Referring to FIGS. 11(a) and 11(b), when the vehicle 100 collides in the pole crash or in the under-ride crash, the collision object (that is, the utility pole 102 or the heavy-duty truck 104) does not collide with the framework of the body of the vehicle 100 but with a relatively soft part of the body. This causes the body to be deformed to a greater depth after the front part of the vehicle 100 crashes into the collision object. The characteristic curve of the retardation G of the vehicle 100, however, does not rise immediately as shown in FIG. 11(c). The characteristic curve of the retardation G rises at last when the body is deformed to a greater extent and the collision object collides with a relatively hard part, such as an engine.
Even when an appropriate threshold value is set for the retardation G to ignite the gas-generating agent with the squib at an appropriate timing, the conventional method using only the retardation itself to control the ignition can not effectively determine the ignition timing of the squib since the retardation G does not rise nor exceed the threshold value after a lapse of some time period as shown in FIG. 11(c) in case of pole crash shown in FIG. 11(a) or under-ride collision shown in FIG. 11(b). The same problem arises when the integral value v obtained by integrating the retardation once is used to control the ignition.